Signal combination can be accomplished by means of passive or active combiners. Passive combiners contain no active or nonlinear elements, such as transistors, while active combiners do contain active devices, such as amplifiers, that provide gain. Scattering or S-parameters often describe combiner performance. A four-port combiner has been used to combine signals. Input signals are input into two ports 1 and 4 and are output from the other two ports 2 and 3 depending on the relative phase of the input signals. The paths between the two inputs ports 1 and 4 and paths between the output ports 2 and 3 are isolated with minimal energy transmission between the ports. The operation of a four-port passive combiner can be described by a generic combiner S-parameter matrix.
      [                            α                          γ                          δ                          β                                      γ                          α                          β                          δ                                      δ                          β                          α                          γ                                      β                          δ                          γ                          α                      ]     
The S-parameters can be described with linear magnitudes and phase, rather than in dB and phase where each of the parameters listed is actually a complex number. The input match parameters α for each of the four ports are usually small in magnitude, on the order of 0.1, indicating a good signal match. The isolation parameters β are also relatively small in magnitude, such as 0.03, indicating a good port isolation. The combining parameters γ and δ can have large or small magnitudes, depending on the specific requirements. In a slightly lossy 50% combiner, for example, γ and δ would each be slightly less than 1/√2 in magnitude. The S-parameters also contain phase information. A passive reciprocal combiner requires that Sij=Sji, where i is the output port and j is the input port. Also, the passive combiner cannot produce more RF output power than is input to the combiner. Mathematically, this means that the sum of the squares of the magnitudes of any S-parameter column or row must be less than one for a lossy case or equal to one in an ideal case. The power conservation requirement of passive combiners allows for only limited tradeoffs in combining values.
An active combiner has been built using several field-effect transistors (FETs). The combiner is built by using FETs connected as transmission gates. In the FET active combiner, the signal path is from the source to the drain or from the drain to the source through the transistor. Feedback through other FETs and resistors is used to provide the combiner isolation. By constructing a fully active combiner through the use of FETs, the input and output paths are all made to be non-reciprocal. The FET active combiner does not meet the requirements of some applications, such as source-pull measurements, that require reciprocity in at least one of the signal paths.
Source-pull measurements are often conducted to investigate the stability of power amplifiers. A typical source-pull circuit requires a directional coupler having a variable load at port 1, dummy load at port 3, a signal source at port 4, and a device under test (DUT) at port 2. The directional coupler isolates the input signal and the variable load when applied to the port of the device under test. Ideally, the variable load that should be applied to the DUT source should have a reflection coefficient ρ that varies in magnitude from 1 indicating a short or an open condition, to 0 indicating a matched load. Because of power conservation restrictions between ports 1 and 2, the directional coupler exhibits a small but significant insertion loss that then reduces the range of the reflection coefficient ρ that can be applied to the source port of the DUT. For example, an ideal 10 dB coupler would reduce the maximum applied reflection coefficient ρ to 0.81. The path from the variable load to DUT source port must also be reciprocal. However, well-designed DUTs, such as amplifiers, are generally stable for relatively large values of the reflection coefficient ρ. These active circuits would tend to oscillate for extreme values of the reflection coefficient ρ, such as those larger than 0.8. A passive combiner with 20 dB coupling could be used to extend the range of available the reflection coefficient ρ. However, a ten times more powerful signal source is required for the test setup. These larger power signal sources can be relatively expensive.
Circulators have been used to translate signals from one port to another port. The operation of a three-port circulator can be described by a generic circulator S-parameter matrix.
      [                            α                          β                          γ                                      γ                          α                          β                                      β                          γ                          α                      ]     
The input match parameters for each port α are relatively small in magnitude. The forward S-parameters S21, S32, and S13 are relatively large in magnitude and less than one, while the reverse S-parameters S12, S23, and S31 are relatively small in magnitude, similar to the isolation parameters β of the combiner. The passive circulator is a non-reciprocal device, modeled by Sij not equal to Sji, that must still satisfy the same power conservation law as does the passive combiner. However, a realistic circulator also has approximately 0.5 dB of insertion loss at γ=0.944 in each of the forward paths.
Circulators are often used at the front end of a transceiver system that contains only one antenna port. Examples of such systems include radar transceivers, cellular phones, and other wireless devices. The circular can have three ports 1, 2, and 3, where a signal can circulate from port 1 to port 2, or from port 2 to port 3, or from port 3 to port 1. In an exemplar configuration, the circulator port 1 is connected to an antenna, a receiver is connected to port 2, and a transmitter is connected to port 3. The circulator allows the transmitted signal to travel, for example, from port 3 to port 1 and then into the antenna. During transmission, the circulator isolates the sensitive receiver attached to port 2 from the large transmitter signal at port 1. This isolation value is on the order of 0.1 units. The received signal is input from the antenna into port 1 and travels through the circulator to port 2. The received signal is thus isolated from the transmitter port 3. The circulator, while performing a needed operation, degrades the signal strength during transmission and reception. The circulator causes a loss in the outgoing signal from the transmitter amplifier. This signal loss reduces the range that the radar can detect objects, or that a wireless device can communicate. Then, on reception, the circulator introduces a loss in the receiver path. Because this receiver loss is placed before the low-noise amplifiers, the loss value adds directly to the noise value of the receiver path, thus degrading the magnitude of the minimal detectable signal.
Circulators also have a certain size and weight that is determined by the frequency of use. When the circulator is used in a space-based phased array application, the total weight of the circulators can be large. Also, a goal of building wireless devices is to reduce the profile of the circuitry and hence the external packaging as much as possible. The design of active circulators and active quasi-circulators are typically large in size. The active circulator designs are rotationally symmetric, while active quasi-circulators are not. The quasi-circulators have only one orientation that can be used in a transceiver application. Typical circulator architectures add a significant amount of noise in addition to the noise introduced by the low noise amplifier that is usually the first element in the receiver path. The best architecture of a quasi-circulator has a 3 dB noise figure. The output powers in some circulator designs are limited by the presence of active devices at the antenna port. The circulator architectures sacrifice both output power and noise figure to achieve as near as possible circulator function. These circulators operate with high noise figures, low output signal levels, are heavy and have large profiles. These and other disadvantages are solved or reduced using the invention.